Arboriculture

arboriculture, cultivation of trees, shrubs, and woody plants for shading and decorating. Arboriculture includes propagating, transplanting, pruning, applying fertilizer, spraying to control insects and diseases, cabling and bracing, treating cavities, identifying plants, diagnosing and treating tree damage and ailments, arranging plantings for their ornamental values, and removing trees. The well-being of individual plants is the major concern of arboriculture, in contrast to such related fields as silviculture and agriculture, in which the major concern is the welfare of a large group of plants as a whole.

The basic principles and objectives of arboriculture are of ancient origin. Early Egyptians transplanted trees with a ball of earth and originated the practice of shaping the soil around a newly planted tree to form a saucer to retain water, both still practiced. About 300 bc the Greek philosopher Theophrastus wrote Peri phytōn historia (“Inquiry into Plants”), in which he discussed transplanting of trees and the treatment of tree wounds. Virgil’s Georgics portrays Roman knowledge of tree culture. The English horticulturist John Evelyn, in his Sylva, or a Discourse of Forest-trees, and the Propagation of Timber (1664), offered advice on pruning, insect control, wound treatment, and transplanting.

Trees or plants may be propagated by seeding, grafting, layering, or cutting. In seeding, seeds are usually planted in either a commercial or home nursery in which intensive care can be given for several years until the plants are of a size suitable for transplanting on the desired site. In soil layering, the shoots, or lower branches of the parent plant, are bent to the ground and covered with moist soil of good quality. When roots have developed, which may require a year or more, the branch is severed from the parent and transplanted. In an alternative technique, air layering, the branch is deeply slit and the wound covered by a ball of earth, moss, or similar material. The ball, enclosed in a divided pot supported from underneath, or in a sturdy paper cone, is kept moist. As in soil layering, the branch is severed and transplanted after roots have developed. Root cuttings can be used for propagating trees that do not normally produce roots from stems. Tree species such as willow and poplar that sucker, or send up shoots readily, are usually propagated from stem cuttings. Cuttings are made from deciduous plants during dormancy, preferably from the terminal growing shoots of the current season. Pieces 6 to 10 inches (15 to 25 centimetres) long with two or more buds are tied in bundles and stored in damp sand or moss for callus formation before planting in prepared beds. Root formation may be stimulated by application of growth-promoting chemicals or growth hormones.

In treating tree-trunk wounds in which large areas of bark are torn away, the bark around the wound is trimmed back to sound tissue and, at the top and bottom of the injury, trimmed to form a pointed ellipse of the wound area. The exposed wood is covered with wound dressing material, protecting it from wood decay fungi.

Flexible cables (guys) or rigid braces are used to support recently transplanted trees until the roots become established, or to lessen the danger that a tree with a weakened root system will be blown over by the wind; bracing is also used to support unduly long or heavy branches, to prevent splits developing at branch forks, or to permit healing of splits already developed.

Cavities in trunks, caused by decay-inducing fungi, may be treated with antiseptic dressing and left open, with drains installed at the bottom, or filled with concrete or other material after removal of the decayed wood. See also graft; pruning; transplant.